You can have the exact same thing in Rockbox. All you need is to have a .cfg file named "fixed.cfg"(no quotes) in your .rockbox folder. It only needs to have one line in it. Mine is "Volume: -26". That boots the player at that volume every time.

If I want to quickly boost the volume and turn off all the processing so the car sound system will play properly I "play" the In The Car.cfg file I saved after I set that up. When I turn on the player again after using it in the car the fixed.cfg file adjusts the volume back to -26 again.

There's a lot that can be done in Rockbox on the user side without waiting for someone who knows how to code to write a patch and fight to get it accepted. You could wait for that or take a minute to create a fixed.cfg file and deal with it yourself. That's the type of thing I meant when I said I learned how to take care of my hearing without waiting for others.